In decades Peer Instruction (PI) has been confirmed that it can improve students' conceptual understanding. Anyway the main problem for using PI is an audience responding system which is required for gathering students' answer, to enhance the learning process of PI instead of using Clickers which cost about 40 USD per item. In this work we decided to use Plickers: the free application in smartphones. The purpose of this research is to develop the students' conceptual understanding in work and energy by using PI strategy. The sample for this study came from the different academic year at Ubon Ratchathani University, Thailand. There were 50 first year chemistry students who have enrolled in introductory physics course in academic year of 2016 and 119 first year engineering students who have enrolled in introductory physics course in academic year of 2017. The pre-post test was the conceptual understanding test in work and energy; there were 6 items with 4 choices. The items were a part of the Multiple-choice Test of Energy and Momentum Concepts developed by Chandralekha Singh and David Rosengrant. The pre-post score had escalated about 13% (chemistry students) and 25% (engineering students). Furthermore both groups of students also had a positive attitude to teaching and learning physics with PI and Plickers approach.